What is the role of exchange vessels?

The role of exchange vessels, primarily the capillaries, is to facilitate the exchange of gases, nutrients, and waste products between the blood and surrounding tissues. This occurs at the cellular level where oxygen and nutrients from the blood diffuse into the tissues, while carbon dioxide and metabolic waste products move from the tissues into the blood.

Capillaries are uniquely structured with thin walls composed of a single layer of endothelial cells, allowing for efficient and rapid exchange. This is essential for maintaining homeostasis within the body, as tissues depend on a continuous supply of oxygen and nutrients for cellular metabolism and to remove waste to keep the internal environment stable. Therefore, the key function of exchange vessels is precisely to permit this crucial exchange, making option B the correct answer.

Other options focus on different physiological roles that are not directly associated with the primary function of exchange vessels. For example, transporting blood away from the heart refers to arteries, regulating blood pressure involves the entire circulatory system and often includes arteries and veins, while controlling heart rate is a function primarily managed by the nervous system and hormonal signals rather than the exchange vessels themselves.